1. Field of the Invention
The present invention relates to a method for rapidly mixing ceramic slurries for use in forming casting molds.
2. Description of the Prior Art
Investment casting is a well-known process for making metal articles of complex shape. Generally, in making a mold for investment casting, an expendable pattern, such as wax, in the shape of the article to be made is dipped in a slurry of ceramic particles, dusted with dry ceramic particulate and dried, the sequence being repeated until the desired thickness for a mold wall is obtained. Thereafter, the wax pattern is removed, such as by melting, and the ceramic layers are heated for consolidation into a strong mold to be used in casting.
The ceramic slurry used in forming the mold about the pattern is usually a suspension of insoluble ceramic powders, such as zircon, alumina, fused quartz and the like, in a binder liquid, such as colloidal silica. The ceramic powders in the slurry are provided in a range of particle sizes to achieve proper mold strength and surface finish. For example, in a typical zircon slurry, the zircon powder may include -600 mesh, -325 mesh and -140 mesh particles in selected proportions depending on the properties desired in the casting mold. U.S. Pat. No. 3,933,190 illustrates a ceramic slurry having alumina powders of various particle sizes suspended in an aqueous solution. Of course, the quality of the ceramic slurry into which the pattern is dipped is a key factor in determining whether a high quality casting mold will be eventually produced. To this end, such slurry properties as viscosity, density and temperature must be carefully controlled within critical limits during and after slurry mixing.
The usual practice for preparing such ceramic slurries involves placing the liquid binder ingredient of the slurry in a cylindrical container having a mixing impeller therein, the impeller having the general shape of an outboard motor propeller. The various ceramic powder ingredients are then added to the colloidal silica in sequence according to the surface area to weight ratio of the powder, the powder having the highest such ratio being added first and so on. For example, in making the zircon slurry mentioned above, the -600 mesh zircon powder is added first, the -325 mesh powder second the -140 mesh powder third. During such additions, the colloidal silica is slowly stirred. The sequential addition of each powder ingredient is a lengthy step in the process, in many cases requiring several hours for each powder ingredient. Such slow, sequential powder additions are necessary to ensure that the fine, difficult-to-wet powders are satisfactorily wetted be the colloidal silica and that minimal settling of the particles occurs. In addition, such slow additions are necessary to preclude entrapment of air in the slurry. Air entrapment lowers the density and raises the viscosity of the slurry which, as already mentioned, must be maintained within critical limits to assure production of high quality casting molds. Nevertheless, some air entrapment is inevitable during mixing due to the vortex action around the impeller shaft. Consequently, standard prior art practce is to stabilize the ceramic slurry after preparation to allow such entrapped air to escape from the slurry. After stabilization, the slurry is finally ready for use in forming casting molds. As a result of the process steps involved, the prior art practice for mixing ceramic slurries is very time-consuming; requiring a total preparation time, including ingredient additions, mixing and stabilization, from 24 to 36 hours and more in many cases. During this time, numerous foundry personnel must be present to ensure porper slurry preparation.